Known chairs of the adjustable type are extremely comfortable, due in large part to the practically vertical position of the back rest. They provide firm support for the user in the starting or working position and, after the back rest has been tilted backward, they also permit more relaxed seating. When positioning the back rest forward or backward in known chairs, the upper body of the user both pivots and undergoes forward or rearward movement. Since the back rest undergoes similar movement, relative movement between the back of the user and the back rest is avoided or at least reduced to an insignificant extent.
Other types of chairs are constructed in a manner such that as the seat member and back are tilted, the back of the user (that body part in contact with the back rest) experiences significant relative movement with respect to the back rest of the chair.
As shown in European Patents 85,670 B1, 176,816 A1 and German Patent 2,916,897 A1, this compensates for the relative movement between the body of the user and the back rest may be solved in different ways. The chair may be designed as a synchronized chair whereby a angular movement of the seat support causes a backward tilt of the back panel, but through a greater angle of movement. See, for example, FIGS. 1 and 3 of European Patent 85670. The seat support may also be mounted on a bearing block in such a way that it must be moved forward as the back is tilted rearward. Compare the relative positions of the seat supports in FIGS. 1 and 2 of German Patent 29 16 897. As exemplified in known chairs, a relatively complex support and pivoting apparatus is desirable for changing the slope of the back rest. However, such mechanisms do not always function satisfactorily, i.e., reliably and free of trouble.
Multiple linkage levers such as those described in European Patent 176,816 A1 or other transmission lever pairs and engaging levers, as described in European Patent 85,670 B1, are used in such cases. In each case, many parts are needed to achieve the adjustment feature. The same thing is also true of a chair according to German Patent 2,916,897 A1, where the back rest is movable downward on the back when the seat is moved forward for more reclined seating.
Another disadvantage of these known chairs is that in certain of them, the adjustment attachment points must permit lateral displacement movement as well as pivoting movement of the connected parts. Therefore, the design of these attachment points is usually complicated and less reliable in operation.